When my Outlook pst file exceeded 20Gb, funny things occurred to my Outlook 2010. Read on some tech forums that Outlook doesn’t handle large pst file, and when the pst file reaches that size, it can’t be compacted.
I’ve been using Yahoo! Mail for registering to online services (because the mailbox size is unlimited so it can withstand spams). The massive amount of emails to my Y!Mail caused the pst file to gorge.
Gmail allows good controls of POP3. Outlook mail (the new hotmail) more obscure. Y!Mail POP3 is very limited e.g. no option to "download arriving emails only". Therefore, ALL emails to Y!Mail are downloaded on every single device when I connect Y!Mail account to an email client.
I am deleting the Y!Mail account from my Outlook and switching back to the web interface.